My British Patent Specification Nos. 1,227,764 and 1,446,071 both describe furnaces for burning solid fuel in which the fuel is delivered down an inlet shaft onto a reciprocatory mechanical grate. The grate advances the fuel through an opening into a combustion chamber for the fuel where a stream of air is passed up through the grate and the fuel bed to sustain combustion. The fuel just upstream of its point of entry is heated by the burning fuel and evolves volatile and gaseous components which are drawn, together with some of the air from the air stream, through the fuel at the bottom of the inlet shaft into ducts in the direction opposite to that of movement of the fuel. This mixture of air and volatile and gaseous components is drawn through the duct and passed up through the grate and fuel bed in a hot region of the latter beyond the air stream. While such a furnace is in general very effective in burning coal efficiently at high output, it has been found that it runs the risk of "burning back" (that is: burning of fuel in the inlet shaft) especially at low burning rates (e.g. overnight operation).
Particularly with a view to overcoming this problem the present invention provides a high intensity method of burning solid fuel in pieces comprising the steps of:
advancing the fuel along a path having an upstream portion from which air is excluded, metering the height of the fuel leaving the upstream path portion to define a predetermined cross section for the fuel as it leaves the upstream path portion and enters a downstream path portion in a reverberatory combustion space as a thick fuel bed, PA1 passing a first high velocity stream of air from below the path through the fuel bed in a self-sustaining ignition zone, the first stream of air supplying the major portion of the oxygen required for combustion and having upstream and downstream boundaries extending transversely of the path, the fuel being rapidly heated and ignited as it moves through the upstream boundary, the metered height of the fuel bed being such in relation to the speed of advance of the fuel and the distance between the upstream and downstream boundaries that the size of the fuel pieces and the thickness of the burning fuel bed at the downstream boundary is still sufficient to prevent destabilisation of the fuel bed by the first stream, PA1 and passing a second, lower velocity stream of air, sufficient to complete combustion, through the fuel bed, the second stream being diffused over a sufficient length of the path downstream of the first stream so as to be of sufficiently low velocity to avoid entrainment of the burning-out fuel particles from the fuel bed.
Correspondingly, the present invention provides a furnace for burning solid fuel in pieces comprising a reverberatory combustion chamber, a mechanical grate for advancing solid fuel along the grate through a metering opening into the combustion chamber, means for supplying solid fuel to fill the entry to the metering opening, means for preventing air from entering the solid fuel upstream of the passage, a first set of air channels through the grate for passing a high-velocity stream of air up through the fuel bed on the grate downstream of the passage entry, the channels defining upstream and downstream boundaries for the first stream and a second set of air channels through the grate for delivering a restricted second stream of diffused air to the portion of the grate downstream of the first channels to complete combustion of the fuel, the arrangement being such that substantially no air from any of the said channels is able to enter the fuel upstream of the metering opening.
If desired, a duct may be included which extends between an inlet in an upper surface of the metering opening and the space beneath the second air channels in the grate for conveying the second air stream, which is here drawn from the first stream together with volatile and gaseous components of the fuel evolved from the fuel as it passes through the opening. With this arrangement, the risk of "burning back" in the fuel inlet is avoided since the second air stream does not enter the fuel upstream of the opening.